In this paper, limit strains of a commercial superplastic 5083 aluminium alloy sheet were investigated under biaxial tensions at an optimal temperature of 823 K. The sheet was deformed under stress ratios of 1, 0.8293, 0.6667, 0.5985 using a superplastic gas pressure forming tester with dies of aspect ratios of 1:1, 4:3, 2:1 and 8:3. An experimental limit strain curve was obtained and its shape was found to be different from that of conventional forming limit curve. An instability model based on the Marciniak-Kuczynski model and the damage mechanics approach was developed to predict the limit strains of the alloy. In the model, an initial geometrical defect and the inhomogeneity in cavity distribution were considered. The predicted limit strains were found to increase with a decrease in geometrical defect size and the degree of inhomogeneity in cavity distribution. It was demonstrated that the model was able to predict limit strains successfully without assuming any initial geometrical defects. The trend of the predicted limit strain curve was found to be in reasonable agreement with the experimental findings.